The present device relates generally to half-turn hooks and more particularly to a half-turn hook fit for use in an automatic sewing machine.
DB-type half-turn hooks have conventionally been employed in automatic sewing machines. FIG. 6 is a top view of a DB-type half-turn hook of the sort mentioned above. As shown in FIG. 6, the DB-type half-turn hook has a shuttle race body 2, a shuttle body, a shuttle bobbin (not shown), which are provided in the shuttle race body 2, and a shuttle race ring 4. A cutout 6 for use in vertically passing a needle and a needle thread therethrough is formed in the top surface of the shuttle race body 2. And a cutout 32 is also formed in the shuttle race ring 4. Moreover, a sliding surface 10 is provided in the rear (upper side of FIG. 6) of the thread dividing portion 8 of the shuttle race body 2, the sliding surface 10 being smoothly finished.
When a machine using such a DB-type half-turn hook is employed for stitching purposes, a needle 16 is positioned between a needle thread loop 12 and a point where a bobbin thread on a bobbin 14 is guided outward. The DB-type half-turn hook in this case is assumed to be such that the needle thread loop 12, the needle 16 and the bobbin 14 are thus related to each other. In FIG. 7, reference numeral 18 denotes a bobbin thread, 20 a needle hole in the needle plate, and 22 a needle location.
FIGS. 8 to 10 illustrate a DB-type half-turn hook in such a state that stitching is started after the thread is cut: FIG. 8 is an elevational view; FIG. 9 a perspective view; and FIG. 10 a partial top view as viewed from the direction of an arrow C. In these drawings, reference numeral 24 denotes a shuttle race cap, 26 a shuttle body, and 27 a needle plate. A needle thread 28 moves along the sliding surface 10 formed on the side of the needle thread 28 of the thread dividing portion 8 of the shuttle race body 2.
When the automatic sewing machine employing such a DB-type half-turn hook is used for stitching, different seams are formed, depending on the cloth-feeding direction. FIG. 11 illustrates seams of different types: FIG. 11(A) shows a perfect stitch; and FIG. 11(B) a hitch stitch; more specifically, the needle thread 28 and the bobbin thread 18 become twisted and entangled in the former case, whereas both threads become entangled in the form a chain in the latter case.
In the case of the perfect stitch, good seams are formed because needle threads are smoothly lifted with excellent tightness of stitches during the stitching operation. In the case of the hitch stitch, on the other hand, a kind of knot is formed by a ring of needle thread 28 and this not only hampers the lifting of the needle thread 28 but also deteriorates the tightness of stitches, though it is fit for turning the course as threads are hardly loosened. The perfect stitch is needed in the beginning of stitching in the case of straight stitch and in order to prevent threads from loosening in the beginning or at the end of stitching, it is preferred to turn the course by means of the hitch stitch. Therefore, the DB-type half-turn hook has heretofore been used.
Notwithstanding, the perfect stitch ought to be made in different stitching directions in cases where the crosswise stitch (zigzag chain stitch), the pattern stitch and so forth are carried out. Since the half-turn hook for use in the conventional automatic sewing machine has been of the aforementioned DB type, a mixture of perfect and hitch stitches occurs in some stitching directions and consequently poses a serious problem in stitching quality.
FIG. 12 illustrates an example different from what is shown in FIG. 7, wherein the needle thread loop 12 is located between the needle 16 and the point where the bobbin thread on the bobbin 14 is guided outward. A hook having the needle thread loop 12, the needle 16 and the bobbin 14 in this relationship to each other is called a DP type. The use of the DP-type half-turn hook justifies the perfect stitch in either case of feeding cloth forward or backward. In order to make certain of the kinds of seams prepared at the time of pattern work, a circular seam approximately 120 mm in diameter was formed at a pitch of approximately 2 mm. FIG. 13 shows the test results. FIG. 13(A) refers to a case where the stitching direction is set clockwise (cloth-feeding direction is counterclockwise) and FIG. 13(B) where the stitching direction is set counterclockwise. In FIG. 13, reference symbol S and reference numeral 31 respectively denote a point where stitching is started and a seam. In addition, reference symbol P denotes a range where the perfect stitch is prepared; and H a range where the hitch stitch is formed. In both cases where the stitching directions are set clockwise and counterclockwise, the range H in which the hitch stitch is prepared is limited to approximately 15.degree. and the perfect stitch is prepared in the remaining range of 345.degree. . It is therefore preferred to employ a stitching method covering a wide range of stitching (or cloth-feeding direction); namely, a DP-type half-turn hook even in the case of the pattern stitch.
The present divisor therefore proposes the use of a DP-type half-turn hook for an automatic sewing machine. FIGS. 14 and 15 illustrate a DP-type half-turn hook in such a state that stitching is started after the thread is cut: FIG. 14 is a perspective view; and FIG. 15 a partial top view as viewed from the direction of an arrow D. In FIGS. 14 and 15, like reference characters designate like or corresponding parts or elements in FIGS. 8 to 10. Like the DB type, the sliding surface 10 is formed at the end of the needle thread 28 in the thread dividing portion 8 of the shuttle race body 2 and, the needle thread 28 is caused to move along the sliding surface 10. Therefore, the needle thread 28 may dig into the groove between the shuttle race ring 4 and the shuttle race body 2 because the shuttle race ring 4 is located in the direction in which the needle thread 28 moves. When the thread digs into the groove like this, it may be torn off or soiled. The digging of the thread starts from the surface 34 on which the shuttle race ring 4 abuts against the shuttle race body 2 as well as an end 30 in the direction in which the shuttle body rotates in the cutout 32 of the shuttle race ring 4; this portion is hereinafter called a digging portion.